1. Field of the Invention
This invention relates to a polarimeter and a method for measuring polarisation states of a light beam.
2. Description of the Related Art
Various types of polarimeters enabling to measure all the polarisation components of light, i.e. the four components of the Stokes S vector of light, are known. Notably the article `Multichannel polarisation state detectors for time-resolved ellipsometry` by R. M. A. AZZAM, Thin Solid Films, vol. 234, pp. 371-374, 1993, exhibits techniques consisting in separating a beam to be measured into several beams and in processing then these different beams in parallel. The number n of final beams is at least equal to four, so that the n intensities measured of the final beams enable to access the four states of polarisation of light, i.e. the four components I, Q, U, V of the Stokes S vector.
Thus, AZZAM has described in the article `Division-of-amplitude photopolarimeter (DOAP) for the simultaneous measurement of four Stokes parameters of light`, Optica Acta, Vol. 29, N.degree.5, pp. 685-689, 1982, a division-of-amplitude polarimeter whose principle is as follows: the incident light beam is first of all separated into a reflected beam and a transmitted beam by a beam-separating plate, then into four using two Wallaston prisms.
Such polarimeters enable, thanks to information multiplexing, real-time measurement of the polarisation components of light. Moreover, they require neither modulation nor mobile portion. However, since the separating plate produces interference effects, the properties of the polarimeter highly depend on the wavelength of light. Spectroscopic applications are therefore out of the question and it is generally necessary to conduct monochromatic measurements.
Another shortcoming of this device is its high sensitivity to the angle of incidence of the beam.